1. Field of the Invention
This invention relates to a light emitter control system, and in particular, to an optical element of a light emitter control system that reflects at least a portion of a light beam from a light emitter to a light-sensing device.
2. Background Information
Computer and communication fiber optic systems are now being developed in which optical devices, such as optical fibers, are used as a conduit for modulated light waves to transmit information. In these fiber optic systems, light emitters are used to produce the light that carries the information. The produced light is then directed to and transmitted by the optical fibers.
Typically, two different types of light emitters are utilized with fiber optic systems. These include, in general, edge emitters and surface emitters. Edge emitters, such as edge emitting lasers, typically have a light emitting portion which is located on an edge of a chip, and typically have an active area that may be, for example, half a micron by four microns in size, for a total area of about 2 square microns. In contrast, surface emitters, such as vertical cavity surface emitting lasers (VCSEL), conventionally have an active area that is substantially larger than the active area of an edge emitter. The active area of a surface emitter is typically around 20 microns in diameter, to provide for about, for example, 400 square microns of active area.
The optical power of a light emitter can vary with changes in the operating temperature or age of the light emitter. These variations can result in inconsistent transmissions.
As such, optical power control systems are used to provide consistent optical power of the light emitters, and thus, more consistent transmissions. In these systems, a portion of the light emitted from the light emitter is detected by a light-sensing device, such as a photodiode, for example, and used to generate a control signal having a signal strength proportional to the emitted optical power. The light-sensing device sends the control signal to control circuitry, which controls the optical power output of the light emitter based on the signal strength of the control signal. The light-sensing device varies the signal strength of the control signal in response to changes in the optical power output of the light emitter.
With edge emitters, such as edge emitting lasers, the control signal has been derived from light emitted from a rear facet of the laser, with a rear facet photodiode collecting and converting the rear facet light to the control signal. That is, the light emitted from the rear facet of the laser is monitored and used to control an output of the light emitted from the front facet.
In contrast, with surface emitters, it is conventional to space the surface emitter away from the end of the optical fiber. This space allows a portion of the emitted light to be collected and utilized for monitoring and controlling the output power of the light beam.
For example, with VCSELs, a portion of the light beam may be directed to a light-sensing device, such as a monitoring photodiode, while allowing the remaining portion of the light beam to be transmitted to the optical fiber. This may be accomplished by using a beam splitter, for example. Alternatively, it is also known to provide an angled glass lid of a TO-CAN package to reflect a portion of the light beam to a photodiode, with the photodiode collecting and converting the reflected light to the control signal.
However, the use of the aforementioned beam splitter disadvantageously increases the cost of the assembly, and reduces the signal strength of the emitted light beam available for transmission to the optical fiber.
Further, the use of the known angled lid in an optical power control system has associated problems. Use of an angled lid requires expensive tooling of equipment to manufacture the angled lids and TO-CAN packages. Further, the lid must be positioned at a precise angle relative to the emitted light beam, in order to allow the partial reflection of the light beam while allowing the rest of the light beam to pass therethrough and to the optical fiber. This requires that the lid be positioned using expensive active alignment techniques. Moreover, it has been shown that an increase in the light output power causes changes in the reflectivity of the angled lid, which may prevent the light from reaching the light-sensing device or optical fiber. Thus, the use of an angled, partially-reflecting glass lid is not an ideal monitoring solution.
Therefore, it would be desirable to provide an optical element for a light emitter control system that would overcome the above-mentioned problems.
It is also known to derive the control signal from light emitted from a VCSEL directly onto a photodiode xe2x80x9cflip-chipxe2x80x9d mounted to the VCSEL. For example, in a 1xc3x972 VCSEL array, a photodetector has been flip-chip mounted to one of the VCSELs to monitor its power variations and adjust the power output of the other VCSEL.
xe2x80x9cFlip-chipxe2x80x9d refers to a surface mount chip technology where a chip is packaged in place on a board and then underfilled with an epoxy. Commonly, the chip is attached by placing solder balls on the chip, xe2x80x9cflippingxe2x80x9d the chip over onto the board and then melting the solder. Flip chips are also mounted on glass substrates, such as LCD drives and smart cards, for example, using a conductive paste.
However, flip-chip mounting photodiodes to VCSELs creates a risk that the VCSEL may be damaged due to contact of the surface of a VCSEL with the photo diode. Therefore, it would be desirable to provide an optical element for a light emitter control system that would not contact the surface of a VCSEL.
As mentioned above, it is conventional to provide a space between the light emitter and the end of the optical fiber. However, this space allows the light emitted from the light emitters to diverge. This divergence may cause some of the emitted light not to reach the optical fibers, thus decreasing the efficiency of the transmission. Further, the divergence of the light increases the difficulty in aligning the emitted light beam with the optical fiber. In order to reduce this divergence, and facilitate the alignment process, the light emitter may be moved to be immediately adjacent to, or even in direct contact with, the optical fiber. However, in such an arrangement, there is no space left between the light emitter and the optical fiber, and thus, no light is readily accessible for creating a photodiode signal. Therefore, it would be desirable to provide a light emitter control system which allows a light emitter to be monitored when the light emitter is directly connected to the optical fiber.
It is, therefore, a principal object of this invention to provide a light emitter control system.
It is another object of the invention to provide a light emitter control system that solves the above-mentioned problems.
These and other objects of the present invention are accomplished by the light emitter control system disclosed herein.
In one exemplary aspect of the invention, an optical fiber is positioned immediately adjacent, or directly coupled to an active light emitter using a fiber guide formed on a chip of the light emitter. The fiber guide includes a bore fabricated using photolithographic techniques. Further, the fiber guide structure will preferably have precisely determined bore diameters with straight, vertical walls. It is recognized, however, that this would be difficult to fabricate by way of ordinary lithographic measures. Thus, the present invention broadly contemplates, in accordance with at least one presently preferred embodiment, that special lithographic methods be employed in fabricating a fiber guide.
One conceivable way of accomplishing this task would involve patterning a photoresist using standard photolithographic techniques and using the developed resist itself as the final structure. Such a process provides a simple, inexpensive, yet effective method of fabricating the desired fiber guides.
When the optical fiber is connected to the active light emitter in the aforementioned or similar manner, light from the active light emitter is not available for monitoring the optical power of the light emitter. Thus, in another exemplary aspect of the invention, at least two light emitters are provided, with one of the light emitters serving as a dummy light emitter to control the optical power of an active light emitter.
In a further exemplary aspect of the invention, the light emitters are both VCSELs, and are formed on the same chip. By forming the VCSELs on the same chip, it can be ensured that their rates of degradation and their temperature performance will be substantially similar. Moreover, forming the VCSELs immediately adjacent to each other, for example separated by about 250 to 500 microns, further helps to ensure similar performance characteristics. Nevertheless, it is contemplated that the concepts of the present invention may be utilized with other types and configurations of light emitters without departing from the spirit and scope of the invention.
In another exemplary aspect of the invention, the light emitted from the dummy light emitter is detected by a light-sensing device, such as a photodetector. An optical element is used to transmit the light from the dummy light emitter to the light-sensing device. For example, in one exemplary aspect of the invention, the optical element is tailored as an inverted cap that sits over the dummy light emitter and the light-sensing device. The optical element will collect and reflect at least a portion of the light emitted from the dummy light emitter to the light-sensing device, for controlling the active light emitter.
The optical element may be molded from an optical grade polymer. This allows the optical element to be manufactured in an inexpensive manner. However, it is also contemplated that the optical element be formed of other materials without departing from the spirit and scope of the invention. Further, to increase the reflectivity of the optical element, in another aspect of the invention, the surface, for example the exterior surface, of the optical element may be coated with a reflective material, such as a reflective metal.
In another exemplary aspect of the invention, the optical element includes a plurality, for example, two or three reflecting surfaces arranged at angles relative to each other. The reflecting surfaces can be tailored and arranged to help direct the light emitted from the dummy light emitter to the light-sensing device.
It is not necessary that all of the light emitted from the dummy light emitter be received by the light-sensing device. That is, the light-sensing device need only detect a portion of the light emitted from the dummy light emitter in order to control the optical output of the active light emitter. As such, the precise configuration and placement of the optical element is not critical. Thus, the optical element can be easily manufactured, and inexpensively placed.
In another exemplary aspect of the invention, the optical element is tailored so that the light emitted by the dummy light emitter is scattered and dispersed. This can be accomplished through the placement of the plurality of reflecting surfaces, for example. This allows for the creation of an optical light spot that may be significantly larger than the active area of the light-sensing device. Thus, this configuration allows the optical element to be positioned without a high regard for positional tolerances, using passive alignment techniques, for example, which reduces manufacturing costs.
In a further exemplary aspect of the invention, the outer surface of the optical element has a relatively smooth region that allows the optical element to be picked up using a conventional vacuum pick-up placement device. This configuration facilitates the manufacturing of the device, by allowing automation to pick-up and place the optical element over the light emitters and light-sensing device.
In another aspect of the invention, the optical element is positioned over the dummy light emitter, so that a gap exists between the lower edges of the optical element and the surface of the dummy light emitter. That is, the optical element does not directly contact the dummy light emitter. By providing a gap, for example of a few hundred microns, it can be ensured that the optical element will not damage the fragile surface of the light emitter.
Further, in another exemplary aspect of the invention, the optical element could be provided with feet that rest directly upon the photodiode chip, and which are tailored so that the optical element is positioned over the dummy VCSEL with the desired gap. The feet could then be fastened to the photodiode chip using a UV curable epoxy, for example. Alternatively, instead of feet, the gap could be formed by depositing the UV curable epoxy, for example, to a thickness that would provide for the desired gap.
In another exemplary aspect of the invention, instead of having a molded cap-shaped optical element, the optical element could be configured as a somewhat rounded glob of transparent epoxy, for example. The epoxy (or other suitable material) could be easily deposited over the light emitters and the light-sensing device while in a semi-liquid state, and then allowed to cure. In use, some of the emitted light from the dummy light emitter will be internally reflected within the optical element to reach the light-sensing device. It is believed that the total light reaching the light-sensing device will be less than when using a molded optical element, as described above. Nevertheless, it is further believed that this arrangement will provide sufficient light to the light-sensing device to monitor and control the optical power of the active light emitter, as will be described. Moreover, this aspect of the invention would be easy to implement, and eliminates one of the two-steps of the earlier-described aspect of the invention, i.e., the optical element will be self-fastening to the light-sensing device and the light emitters.
In an exemplary explanation of the use of this arrangement, a portion of the light emitted from the dummy light emitter is collected and reflected by the optical element to the light-sensing device. The light-sensing device uses the detected light to generate a control signal having a signal strength proportional to the optical power of the emitted light. The light-sensing device sends the control signal to control circuitry, which controls the optical power output of both the active and dummy light emitters based on the signal strength of the control signal.
Thus, the light-sensing device varies the signal strength of the control signal in response to changes in the optical power output of the dummy light emitter. Further, the dummy light emitter, light-sensing device and control circuitry form a closed loop system used to control the optical power output of the active light emitter.